The utilization of ceramic ink compositions to form decorative patterns on the surfaces of glass sheets is well known. Ceramic ink compositions typically contain one or more metal oxides, a low melting point glass frit, and a vehicle for applying the ink composition to a surface. The one or more metal oxides act as coloring agents for the ink while the glass frit acts as a bonding agent to embed the one or more metal oxides into the surface of the glass sheet to be colored. The vehicle is typically an ultraviolet (UV)-curable composition which, upon exposure to UV radiation, polymerizes and forms a secure film upon the glass surface so that the ceramic ink coated surface can be handled easily. Upon exposure of the coated glass surface to heat, the UV-curable vehicle is burned off while the glass frit is melted so as to imbed the one or more metal oxides into the glass surface.
It is oftentimes desirable to apply a fiberglass-forming die to the glass sheet as it is being heated in order to form the glass sheet to a desired shape. When the heated glass sheet is covered with a ceramic ink, the ceramic ink has a tendency to stick to the fiberglass-forming die which can result in inconsistencies in the ceramic ink coating process. One attempt to solve this problem is disclosed in U.S. Pat. No. 4,684,388 wherein finely divided zinc metal powder is contained within the ceramic ink so as to prevent the ink from sticking to the fiberglass-forming die during the heating process. However, it has been found that the addition of zinc powder to a ceramic ink composition based on a UV-curable vehicle causes the UV-curable materials to polymerize prematurely, resulting in instability of the zinc-containing composition. Due to the instability of such a zinc-containing UV vehicle-based ceramic ink, the zinc powder must be mixed with the ceramic ink immediately prior to application. The handling and mixing of zinc powder on the site of application is problematic in that the zinc powder is difficult to homogeneously incorporate into the ceramic ink. The zinc powder also poses health and fire hazards through the generation of airborne dust.
A need exists for a ceramic ink composition which can be applied to a glass sheet so as to not stick to a fiberglass-forming die during the heating and shaping of the glass sheet and which does not require the handling and mixing of zinc powder immediately prior to application of the ceramic ink.